(1) Field of the Invention
The present invention relates to an improved unit which in operation supplies combustion air to a furnace and/or to a hot water heater from outside of a building and which when not in operation is closed to the outside of the building. In particular the present invention relates to a unit which preheats combustion air to the furnace and which continues to operate after the furnace and/or heater cease to generate heat to insure venting of the hot flue gases from the furnace or hot water heater.
(2) Prior Art
My U.S. Pat. No. 4,262,608 is believed to show the closest prior art. As shown in FIG. 10, the motor 16 in unit 10 operates while a gas furnace is operating. The unit 10 operates very well; however, there is a concern that when the dampers 39 and 40 close after the operation of the unit, there may be trapped hot flue gases which are prevented from escaping from the furnace or hot water heater with the result that they can leak from the furnace or hot water heater into the basement or other room housing this equipment. The dampers 39 and 40 are necessary to prevent cold outside air from coming into the building when the motor 16 is stopped. Various solutions were conceived; however, none provided positive control over the operation of the motor 16 in unit 10 to allow safe evacuation of the flue gases after the furnace and/or hot water heater burners stopped producing heat.